Which phase transition requires more heat, melting or freezing, if the mass of water/ice in both cases is #"35.0 g"#?

1 Answer
Jan 22, 2017

Neither.


Recall that the freezing point IS the melting point. That means at #0^@ "C"#, the melting/freezing point of water, we really have:

#q = nDeltaH_"trs"#,

where:

  • #q# is the heat absorbed to melt the solid, or heat released due to freezing the water.
  • #DeltaH_"trs"# is the enthalpy for the phase transition.
  • #n# is the #"mol"#s of water in this process.

So, if the masses are both #"35.0 g"#, then the #"mol"#s of water are equal in either case:

#q_"frz" = n_1DeltaH_"frz"#

#q_"mlt" = n_2DeltaH_"mlt"#

where #n_1 = n_2 = n#.

But since #DeltaH_"mlt" = -DeltaH_"frz" = "6.02 kJ/mol"#, we really have that #|q_"frz"| = |q_"mlt"|#. Therefore, both involve the same amount of heat.

Which #q# is positive, #q_"frz"# or #q_"mlt"#?